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    <!-- 数组扁平化 -->
    <script>
      let arr = [1, [2, [3, 4, 5]]]
      function flatten(arr) {
        let result = []

        for (let i = 0; i < arr.length; i++) {
          if (Array.isArray(arr[i])) {
            result = result.concat(flatten(arr[i]))
          } else {
            result.push(arr[i])
          }
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        return result
      }
      console.log(flatten(arr))
    </script>
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